Syndeformational fluid trapping in quartz: determining the pressure‐temperature conditions of deformation from fluid inclusions and the formation of pure CO2 fluid inclusions during grain‐boundary migration

Abstract

Abstract Observations and microthermometric data on fluid inclusions from a terrane that underwent deformation following peak metamorphic conditions show that grain‐boundary migration recrystallization favours the entrapment of carbonic inclusions whereas microfracturing during brittle deformation favours the infiltration and eventual entrapment of aqueous fluids. Our results imply that pure CO2 fluid inclusions in metamorphic rocks are likely to be the residue of deformation‐recrystallization process rather than representing a primary metamorphic fluid.Where the temperature of deformation can be deduced by other means, the densities of fluid inclusions trapped during recrystallization, which we call recrystallization‐primary fluid inclusions, can be used to constrain the ambient pressure during deformation. Using these constraints, the data imply that the post‐metamorphic Hercynian exhumation in Sardinia brought rocks at 300° C to within 3km of the surface. This conclusion is similar to that described for the rapidly uplifted Southern Alps in New Zealand.